The present invention relates to a panoramic ion detector. This detector may be used for the detection of charged particles, particularly in mass spectrography.
For a considerable period of time, the only charged particle detector having a panoramic collection has been the photographic plate. This type of detector has certain advantages such as a good spatial resolution which is below 10 microns and a relatively good detection sensitivity, of the order of 10.sup.6 ions/mm.sup.2.
However, this photographic detector has serious disadvantages and particularly difficulties of production and use whereby developing, recording and the calibration curves of the photographic plates involve a considerable amount of time.
Certain physical phenomena which produce charged particles last a very short time. In certain cases, the study of these phenomena require the use of a particle detector with a very rapid response. In this case, it is known to use a photomultiplier whose response is very rapid. This photomultiplier has a collection and detection window which is only sensitive to direct photon impact. In this case, it is therefore necessary to interpose a charged particle-photon conversion system between the emission source of these charged particles and the photomultiplier. This conversion system generally comprises a scintillator which effects the conversion in a very short time (approximately 1 nanosecond). Therefore, this conversion does not significantly modify the response speed of the photomultiplier, but this system can obviously only be used when the particles to be detected are electrons.
When the charged particles are positive ions for example, it is then necessary to have a supplementary conversion system making it possible to convert the positive ions into electrons, so as to be able to effect the electronphoton conversion by means of the scintillator.
It is known to convert the ions into electrons by different means, all of which involve the use of methods for the reemission of electrons by a surface bombarded with positive ions. The following are examples of such methods:
a detector using an electrical conversion wire, which captures or traps the positive ions passing in its attractive electrical field, then reemitting secondary electrons as a result of this impact; PA1 a detector which uses a microchannel plate whose ends have a significant potential difference. These microchannels capture or trap the ions and convert them into electrons, while also having the effect of amplifying the electrons. Such a plate makes it possible to obtain a high speed panoramic ion--electron--photon converter system. The microchannels are juxtaposed in the plate and ions--electron conversion takes place when an ion to be detected strikws the inner wall of a plate channel. In general, this microchannel plate is placed at an angle of incidence of 20.degree. relative to the ion beams. The plate has thousands of very small channels, for example with a diameter of 12 microns. These channels are very close to one another but, despite this proximity, joints which are blind to the beam of ions exist between each of the channels. Thus, besides a loss of information, the surface for converting the ions into electrons is discontinuous, with a resulting loss of spatial resolution. The ions which have struck the inner wall of a channel are converted into electrons, which are multiplied by rebounding in the channel. The electrons are extracted from the channel by means of the potential difference .DELTA.V which exists between the inlet face of the microchannels and their outlet face. In general, the inlet face is raised to earth potential, while the outlet face is brought to a positive potential of a few kilovolts (+3 kV for example). The amplification factor of the electrons is in particular dependent on the potential different .DELTA.V applied to the microchannels, the position of the ion trajectory relative to the corresponding channel, the local intensity of the beam of incident ions, etc. The configuration of the electron trajectories in the channels is dependent on the potential difference .DELTA.V. The number of impacts between the electrons and the walls of the channels is also dependent on this potential difference .DELTA.V. Consequently, the amplification is itself dependent on .DELTA.V. In order to have maximum detection, it is conventional practice to use plates with a maximum amplification factor. As a result, the plates operate under saturation conditions, no matter whether it is a question of detecting 1 ion or several ions with the same signal amplitude. The existing magnetic field does not modify the spatial resolution due to the existence of the channels. However, it plays an essential part in the amplification due to its non-homogeneous spatial distribution and its time non-reproducibility. Such a plate-type converter consequently does not make it possible to effect a proportional conversion of the ions into electrons. However, it should be noted that it has a very high sensitivity corresponding to a detection capacity of 1 ion. In general, it is standard practice to place a phosphorescent layer behind the plate-type converter which makes it possible to convert the electrons from the microchannels into photons. This phosphorescent layer is brought to a positive potential, whose absolute value is higher than that of the outlet face of the microchannels. Following the said phosphorescent layer, light guides, such as optical fibres, make it possible to capture the photons emitted by the phosphorescent layer and to channel them up to a reading window. The study of the spectrum of the photons is performed at the ends of the optical fibres. These fibres are in fact arranged in such a way that they retain a certain spatial resolution of the initial ionic phenomena. Reading of the spectrum takes place for example by means of a Vidicon sensing camera. However, it is necessary for the ion beams to be collected continuously or at least semi-continuously with a slow repetition level at the detector inlet in order to permit the sensing of the camera.
The conversion of the ions into electrons is not proportional, nor is the intensity of the photon spectrum lines obtained and the complete panoramic detector does not have a proportional response.
Another known detector making it possible to convert ions into electrons followed by a conversion of electrons into photons comprises electrical tapes or wires which are brought to a negative potential and which permit the conversion of ions into electrons. Each of the wires or tapes is associated with a microscintillator, making it possible to convert an electron into a photon. As in the previous case, this detection system has a discontinuous conversion surface. Thus, here again, there is a loss of spatial resolution in the detection of the ions. Moreover, this system requires a plurality of photomultipliers, so that it is scarcely more advantageous than the prior art system with microchannel detectors using a plurality of windowless electron multipliers and consequently its practical realization causes considerable difficulties, as a result of the size of the multipliers.
In order to eliminate this discontinuity in the spatial resolution of detectors, it is known to realize a conversion electrode which is unipolar and has a continuous structure. This electrode is generally brought to a negative potential and the ions strike it without previously being accelerated in the defined space. This negative potential makes it possible to accelerate the ion trapping phenomenon and to eliminate the rebound phenomena during the impact of said ions on the conversion electrode. The secondary electrons emitted as a result of impacts are channelled by an electrical field which is perpendicular to the plane of incidence of the ion beams. These electrons are then collected on a photographic plate after passing through a diaphragm brought to earth potential.
Although such a system makes it possible to obtain a continuous spatial resolution, it does not permit the elimination of the photographic plate. This plate can be replaced by a detector with a scintillator and photomultiplier, but in mass spectrography the use of a detector for multiplication and in particular electron--photon conversion is made impossible due to the overall dimensions of the multiplication elements which are incompatible with the coils for creating magnetic fields.
Another type of known detector involves placing a disk-shaped scintillator in the path of a single ion beam. In this detector the scintillator is covered by a thin metal film. The scintillator is only sensitive to high energy electrons and is insensitive to the positive ion to be analyzed. An accelerating electrode having an opening is placed between an ion source and the scintillator and makes it possible to convert the ions into electrons.
Unfortunately, this detector does not prevent the entry of electrons which do not come directly from the ion--electron conversion and does not permit the spatial analysis of several ion beams.